Fixed union with 'Union' type from 'typing' to support python<3.10

[chdominguez]

I have a project built with python 3.9, and initially molviewspec was not working. I found that the nodes.py module was using | notation to type hint unions, and this does not work on python versions prior to 3.10. Thankfully, the typing module includes a Union class to represent such types, and this works also on older versions of python.

Without this change, molviewspec simply does not work on older versions of python than 3.10

[JonStargaryen]

Hi Christian,

thanks for your interest and contribution. Other parts of the library make use of | as well (specifically types of method parameters in builder.py). Do these cause issues too?

[chdominguez]

On my brief testing with the library that's the only issue I found. But still I think we should change all | for Union just to improve compatibility. I will take a look and update the PR!

[chdominguez]

The issue is not present on builder.py because annotations are imported from __future__ which allows for Unions on python 3.9.

Other important change that I made is to froze the pydantic version to <2 (which was automatically installed when creating a conda environment with the environment.yaml file due to other requirements). This is necessary as pydantic>2introduced some changes on the validator root class which is not backwards compatible with pydantic<2.

[JonStargaryen]

Great, many thanks!